1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer-program product. In particular, the present invention relates to an image processing apparatus, an image processing method, and a computer-program product that can achieve optimum distribution of a dot array in whole color plates for a multi-valued image data containing at least two types of color components, and can prevent color overlay in each color plate.
2. Description of the Related Art
An inkjet recording system has attracted attention as a system for office use, because it can achieve high-speed recording with no need of special fixing processes on so-called plain papers while it can suppress noise caused by the recording to a negligible level. Conventionally, various systems have been proposed, and products thereof have been manufactured and put into practical use. In the inkjet recording system, an inkjet head formed of an ink chamber and a nozzle, communicated with the ink chamber, is used to form images in such a manner that pressure is applied to ink in the ink chamber depending on image information so as to cause ink small dots to be sprayed from out of the nozzle and attached to a recording medium such as a paper or a film. There are serial inkjet printers and line inkjet printers classified according to their head structures. In the serial inkjet printers, an image is formed by scanning a paper with an inkjet head in a paper-width direction (main scanning), and when the scanning is completed one or more times, the paper is conveyed to form a next recording line. On the other hand, in the line inkjet printers, nozzles are formed so as to cover almost the whole area in the paper-width direction, and recording is performed while a paper is being conveyed without scanning motion in the width direction. The latter has advantages in that recording speed is high because one line in the width direction can be formed at one time, but has disadvantages in that a size of the printer increases because a size of the head itself increases, and a manufacturing cost for the head increases because the nozzles need to be arrayed at high density to achieve recording at high resolution. In contrast, the former has advantages in that a cost for the apparatus is low because a relatively small head is used to form images. Therefore, a large number of serial inkjet printers have been put to practical use.
Regarding colors of recording materials used for forming output images in conventional inkjet recording apparatuses, four colors are generally used, which are cyan (C), magenta (M), yellow (Y) being three primary colors employed in subtractive color mixing, and added with black (K). In this operation, images are formed by using recording materials of respective colors after a pseudo halftone process is performed, in which three color components of red (R), green (G), blue (B) of input image data are converted into three colors of C, M, Y or four colors of C, M, Y, K, and the multi-valued image data is converted into binary image data.
As a means for converting multi-valued image data into binary image data, an error diffusion method proposed by R. Floyd et al. is known (see, for example, “An adaptive algorithm for spatial gray scale”, SID International Symposium Digest of Technical Papers, vol. 4.3, 1975, pp. 36 to 37). The error diffusion method is implemented to realize pseudo tone expression by diffusing a quantized error generated at a certain pixel to a plurality of successive pixels. When the pseudo halftone process is performed on multi-valued image data for CMYK four colors, a process is separately performed for each color by using the above-mentioned error diffusion method or the like. In this operation, a binary image generated for each color has excellent visual quality. However, a color binary image in which two or more colors are composited does not always have good visual quality.
FIG. 24 is a diagram illustrating a binary image containing a cyan pixel group, a binary image containing a magenta pixel group, and a binary image in which these images are composited. In the figure, 601 denotes a cyan binary image, 602 denotes a magenta binary image, and 603 denotes a composite image of the cyan binary image 601 and the magenta binary image 602. Each binary image for cyan and magenta is generated by applying the error diffusion method to image data having homogeneous pixel values. We can see each pixel is formed at constant intervals and with good visual quality. On the other hand, in the binary image 603 in which two colors are composited, because positions of cyan pixels and positions of magenta pixels are not correlated, intervals between positions where pixels for respective colors are formed are not uniform. Furthermore, even though the area contains a small number of pixels, some pixels are formed of two colors overlapping each other. As a result, the binary image can be far from having good visual quality.
To overcome the above-mentioned disadvantages, Japanese Patent Application Laid-open No. 2005-136975 discloses a technology performing a halftone process by adding an error caused by quantization to other color components per pixel, and then performing a subtraction process on a color component other than the added color components by centering around the processed pixel position.
However, in the technology disclosed in Japanese Patent Application Laid-open No. 2005-136975, the process is performed per pixel and neighboring errors are not considered. Therefore, although overlapping of dots for different color components can be prevented for each pixel, dot array for the whole image can not optimally be determined.